This invention relates generally to an electronic fuel injection control system and more specifically to an electronic fuel injection control system wherein the pulses controlling the injection of fuel into the engine are frequency modulated and asynchronous with engine speed.
In conventional fuel injection systems, fuel is metered to the engine according to certain engine parameters which are sensed by suitable sensing means. Typically, the quantity of intake air per cycle of the engine is sensed by a suitable manifold pressure sensor positioned in the intake manifold of the engine. Thus, the fuel is metered in accordance with the sensed manifold pressure, this pressure determining the length of the injection pulse, and fuel is supplied to the engine in synchronism with engine rotation. Typically, the above described system is utilized in connection with a multiple point fuel injection system but may also be applicable to a single point fuel injection system wherein fuel is injected at a single point for multiple cylinders upstream of the fuel charge intake for the engine.
However, in the single point injection for controlling the injection of fuel into the engine as described above, it has been found that stratification between fuel and air occurs whereby, during a given period, a series of fuel pockets occur between pockets of air forming the remainder of the fuel charge. This is due to the long period of on-time and off-time which occurs when a single pulse is utilized to inject fuel into the engine. Further, with a single pulse being injected into each cylinder for each engine cycle and a pulse is missed for any cylinder of any cycle, that pulse occurring at a later time, the engine for that cycle will be 100% lean in that no fuel will be inserted into the fuel/air charge and, upon occurrence of the subsequent pulse, will create a situation of a 100% rich mixture in that two pulses are being fed into the cylinder for an engine cycle rather than one as required.
Further, as is seen from the description above, the control of the fuel being fed to the engine occurs by controlling the pulse width of each pulse being fed to the engine. Accordingly, for small variations from the stoichiometric or other desired operating point, small variations in pulse width will occur. It has been found that a degree of difficulty and inaccuracy enters into the control of the required pulse width or on-time for the injector to achieve a certain operating point when the pulse width modulation system is being used. This difficulty is made more acute when the pulse widths are small, as for example in the idle and light load conditions, and it is these operating conditions which creates the greatest pollution problem with respect to emissions from the engine. However, at high loads the pulse width modulation system is relatively accurate due to the long duration of the pulses being fed to the injector system. However, polluting types of emissions are of no great concern at these operating levels in view of the fact that this point of operation occurs less often in the engine operation.
It has been found that the injector accuracy deteriorates rapidly at pulse widths smaller than 1.5 to 2 milliseconds and it is desirable to select a minimum pulse on-time to be somewhere between 2.5 milliseconds to 4 milliseconds. With the minimum on-time duration selected in this range, it has been found that the injectors will respond with sufficient rapidity to maintain engine fuel flow in sufficient quantities to operate at the stoichiometric point or other selected operating point.
In the patent to Toshi Suda et al, U.S. Pat. No. 3,786,788, issued Jan. 22, 1974, there is proposed a fuel injection apparatus for an internal combustion engine, the apparatus including a throttle position sensor which produces an analog signal representative of the throttle position and thus air velocity to the engine if the configuration of the air conduit is known. This throttle position sensor provides a signal to an astable multivibrator circuit, the output frequency of which varies as a function of variations in the throttle position signal. This output frequency signal is fed to a pulse shaping circuit for modifying the shape of the pulse without altering the frequency of the pulse train.
The output of the shaping circuit is fed to a monostable multivibrator which provides an output pulse train having a fixed on-time and an off-time which varies as a function of the frequency of the pulse train being fed thereto from the shaping circuit. The output of the monostable multivibrator is fed to a current driver circuit which, in turn, is connected to control the solenoid valves associated with the injectors.
This prior system has certain inherent disadvantages in that the control unit for controlling the injection pulses to the injectors utilizes a sensing system which includes only sensing an indication of the velocity of the air flow to the engine. Particularly, there is utilized a throttle position sensor, which sensor generates a throttle position analog signal to control the frequency output of the astable multivibrator described above. Accordingly, there is no provision for sensing the mass of the air flow to the engine.
Further, the aforementioned system disclosed in the Toshi Suda et al patent relates to a multi-point injection system rather than a single point injection system which unduly shortens the pulse duration of each of the injection pulses being fed to the respective cylinders of the engine. Finally, there is no provision in the Toshi Suda et al patent disclosure for modifying the pulse generation circuitry in the event that the pulses become so extremely short in duration as to make accurate control of the injectors a substantial problem.
The system of the present invention has been designed to alleviate the problems noted above. In a preferred embodiment of the invention, a system incorporates a manifold absolute pressure sensor which senses the pressure in the intake manifold of the engine under consideration. The output of the pressure sensor is an analog voltage signal, the amplitude of which varies as a function of manifold absolute pressure. The system further includes a sensor for sensing ignition pulses to provide an analog signal representative of the engine speed. This analog engine speed signal, as is the analog pressure signal, is fed to a multiplier circuit which produces an analog output voltage directly proportional to the mass of the air being supplied to the engine per unit time.
The output from the multiplier circuit is fed to a voltage controlled oscillator, the voltage responsive oscillator producing a stream of output pulses having a frequency which is directly proportional to the analog voltage signal representing the mass air flow. Accordingly, the system as thus described produces a variable frequency signal which is representative of a preselected relationship between the magnitude of manifold pressure and frequency of ignition pulses. However, the pulses from the oscillator are voltage spikes, not the pulses required in a fuel injection system of this type. Accordingly, the output of the voltage controlled oscillator is fed to a pulse generator which is capable of producing output pulses in response to an input pulse, the output pulses each having a duration which is extremely accurately controlled. Also, amplitude of the output pulses from the pulse generator are similarly accurately controlled. From the foregoing, the output of the pulse generator is seen to be a stream of pulses having a fixed duration and a fixed amplitude, the off-time varying as an inverse function of the frequency signal being fed from the voltage controlled oscillator. It is these output pulses which are utilized to control the operation of the injector.
In one embodiment of the system of the present invention, it is contemplated that the injector assembly will include a primary and secondary injector which injects fuel into the fuel system of the engine at a single point. This point may vary from engine to engine depending upon the particular type of fuel system selected for that engine.
In the above referenced Toshi Suda et al patent, there is no teaching of a method or manner in which the control of the injection system may be varied in accordance with the output pulse conditions present at the injectors. For example, if the pulses being supplied to the injectors are sufficiently close together indicative of a high frequency being fed from the astable multivibrator, control of the injectors may be lost due to the fact that the injectors are incapable of operating at the frequency being generated by the multivibrator. Further, there is no disclosure in Toshi Suda et al as to how the output pulse width from the monostable multivibrator may be varied in accordance with any variable features incorporated into the multivibrator.
This analog pressure signal and the analog engine speed signal are designated V.sub.pres and V.sub.rpm and the resultant output analog signal from the multiplier varies as a direct function of the product of the V.sub.pres and V.sub.rpm signals. The multiplier also includes a further input which is fed back from the output of the control circuit to control a divider circuit associated with the multiplier circuit. This function will be explained more fully hereinafter.
The output analog signal from the multiplier circuit, designated V.sub.m, controls a voltage controlled oscillator to generate a frequency signal, the control of the frequency being directly related to variations in either the pressure sensor or engine speed or both. Therefore, the frequency modulated signal varies as a function of the mass air flow to the engine, the mass air flow being related to the manifold absolute pressure and the rotary speed of the engine. These output pulses from the voltage control oscillator are not controlled as to amplitude and pulse duration, which function is performed by a pulse generator which is connected downstream from the voltage controlled oscillator. The pulse generator, when provided an input pulse, will provide an output pulse having a precisely controlled amplitude and pulse duration for the on-time with a variable off-time varying as an inverse function of the frequency being generated by the voltage controlled oscillator. Thus, the duty cycle of the output pulse train from the pulse generator varies as a direct function of the frequency output from the voltage controlled oscillator. This output pulse train is fed through an OR gate to an output terminal connected to the solenoid associated with the injectors, the on-time of the pulses from the pulse generator determining the on-time for the injectors.
With the system described above, there has been provided a frequency modulated control circuit for a single point fuel injection system, the frequency of which is controlling the duty cycle of the pulses being fed to the injectors as a function of the mass air flow to the engine. In this way, the variable operational parameters of the engine are sensed to provide control for the injectors. In engines of the type normally utilizing an injection system, the fuel requirement increases as a function of increased engine load and/or increased engine speed. Accordingly, both engine functions are sensed to provide control for the duty cycle of the pulse train, contrary to certain systems of the prior art.
A problem may arise if the engine is operating under load at high speed and the duty cycle of the output pulses from the pulse generator approaches a preselected percentage, for example, 80%. In this situation, the injectors will be on for a relatively long period of time and would be turned off for an extremely short period of time, whereupon they would again be turned on. With this high duty cycle, it is possible that the inertia of the injector be so great as to cause the injector to fail to turn off or partially turn off and the injectors may unduly wear. Accordingly, the system of the present invention senses the duty cycle of the output pulses being fed to the injectors and, upon the duty cycle reaching a pre-selected value, will operate a duty cycle switch to provide an output signal which is fed back to the multiplier circuit. This output signal operates on circuitry associated with the multiplier circuit to reduce the effective output of the multiplier in response to pressure and ignition pulse changes by a preselected factor, for example, one-half or one-third. The duty cycle switch also generates an output signal which is fed to the pulse generator to increase the pulse length being produced by the pulse generator as an inverse function of the reduction of the output multiplier voltage. For example, if the output multiplier voltage is reduced by one-half for preselected pressure and ignition pulse sensor outputs, the pulse length would correspondingly be increased by a factor of two. In this way, the amount of fuel being fed to the engine is maintained at a constant rate for a preselected pressure and engine speed while at the same time maintaining continuous accurate control over the operation of the injector. In this way the injector life may be extended.
It has been found that additional fuel requirements arise in an engine operating at a low temperature and during cranking. With regard to the cranking situation, a temperature sensitive pulse generation circuit has been provided which is responsive to engine temperature and the cranking condition. The output pulses from this circuit are fed to the OR gate to control the injector during engine cranking operation.
Accordingly, a temperature sensor is provided which produces an output signal corresponding to the engine temperature, this signal being fed to a coolant temperature circuit which generates an analog output signal in the form of a voltage, the amplitude of which is directly related to the engine temperature (V.sub.H.sbsb.2.sub.O) and indirectly related (F.sub.H.sbsb.2.sub.O). This V.sub.H.sbsb.2.sub.O signal is fed to the voltage controlled oscillator circuit to provide a reference voltage for the oscillator circuit to compare with the mass air flow signal V.sub.m, and to a cold start circuit, V.sub.H.sbsb.2.sub.O, which generates output pulses having a preselected length and duration, this duration being greater than the duration of the pulses being fed from the pulse generator to the OR circuit. The cold start circuit also includes an enable signal designated "start crank" which enables the cold start circuit during cranking and inhibits the pulse generator circuit. At the end of cranking, the cold start circuit is inhibited and the pulse generator circuit is enabled.
Accordingly, it is one object of the present invention to provide an improved electronic fuel injection system of the frequency modulated type which is responsive to the mass air flow to the internal combustion engine.
It is another object of the present invention to provide an improved electronic fuel injection system which includes a means for sensing the mass air flow to the internal combustion engine and provide the engine with a plurality of fuel injecting pulses asynchronously therewith, the system further including a means for modifying the mass air flow signal in accordance with the frequency of the pulses being fed to the engine fuel system.
It is still another object of the present invention to provide an improved control for the fuel supply of an internal combustion engine to obtain an optimum fuel-air ratio without synchronizing the fuel supply with the engine speed.
It is still another object of the present invention to provide an improved fuel injection control system wherein the injection of fuel to the internal combustion engine is controlled by means of a frequency modulated pulse train, the frequency of which varies in response to the mass air flow being fed to the engine.
It is still a further object of the present invention to provide an improved fuel injection system of the type described which further includes a means for modifying the injection pulses being fed to the internal combustion engine in accordance with the sensed engine coolant temperature.
It is still another object of the present invention to provide an improved internal combustion engine fuel control system which is inexpensive to manufacture, reliable in use and achieves a desired optimum air fuel ratio .